Small noncoding RNAs, including microRNAs (miRNAs), small interfering RNAs (siRNAs), repeat-associated small interfering RNAs and piwi-associated RNAs, 21 to 30 nucleotides in length, could shape diverse cellular pathways. MiRNAs are sequence-specific regulators of post-transcriptional gene expression, and are believed to regulate the expression of thousands of target mRNAs, with each mRNA targeted by multiple miRNAs. Although it has been estimated that miRNAs could regulate as many as one-third of human genes, high-throughput sequencing data indicates that only a portion of small RNAs in the genome have been identified, and the identification of the remaining small RNAs is critical for understanding the small-RNA- mediated gene regulation. Therefore investigating the role of small RNA in depression is likely to be a fruitful, albeit currently understudied, area of research. Methyl-CpG binding protein 1 (MBD1) is a central player of epigenetic mechanism that is critical for gene expression regulation, chromatin structure establishment, and genomic stability maintenance. We have found that Mbd1-/- mice exhibit reduced adult neurogenesis, impaired learning, increased brain serotonin activity, increased anxiety and depression, and deficits in hypothalamic-pituitary-adrenal (HPA) axis. Given the important roles of adult neurogenesis, serotonin, and HPA axis in depression, MBD1 could play important roles in the pathogenesis of depression. Using our established technologies for small RNA analyses, including miRNA array, quantitative RT-PCR of miRNAs and high-throughput sequencing of small RNA libraries, we have found that the loss of MBD1 alters the expression of specific miRNAs in both adult neural stem/progenitor cells (aNSCs) and medial frontal cortex (MFC). In addition, we have identified several known miRNAs critical for the proliferation and differentiation in aNSCs as well as more than two thousand previously unidentified small RNAs in wildtype aNSCs. Functional crosstalk between epigenetic modulation and small RNA pathway has been clearly demonstrated in other species, and our preliminary data prompts us to investigate whether such crosstalk is the key regulatory mechanism for complex functions and related disorders in mammalian brains. These two linked- R01 applications will test the hypothesis that the expression of small RNAs modulated by DNA methylation- mediated epigenetic mechanism plays critical roles in the etiology of depression disorder. We propose a multidisciplinary approach that will apply our combined expertise to first identify the small RNAs and their mRNA targets in aNSCs that are critical for adult neurogenesis (Aim 1);then test the hypothesis that MBD1 regulates the expression of small RNAs that modulate the expression of the key factors of serotonin and HPA axis (Aim 2);and finally determine whether small RNA-mediated gene regulation is involved in the effect of antidepressants (Aim 3). The success of the proposed work should advance our understanding of the role of small RNAs in the etiology of depression, and provide new targets for further research and therapeutic development.